In order for Altair to know that you want to highlight all items with the same symbol as the selection, you need to provide a fields argument to the selection.

import altair as alt
from vega_datasets import data
import pandas as pd

stocks = data.stocks()
source = (
    stocks.groupby([pd.Grouper(key="date", freq="6MS"), "symbol"])
    .mean()
    .reset_index()
)

hover_select = alt.selection_point(
    name="hover_select", on="pointerover", empty=False, fields=["symbol"]
)
conditional_color = (
    alt.when(hover_select)
    .then(alt.Color("symbol:N"))
    .otherwise(alt.value("lightgray"))
)

alt.Chart(source).mark_line(point=True).encode(
    x=alt.X("date:O").timeUnit("yearmonth").title("date"),
    y="rank:O",
    color=conditional_color,
).add_params(hover_select).transform_window(
    rank="rank()",
    sort=[alt.SortField("price", order="descending")],
    groupby=["date"],
).properties(
    title="Bump Chart for Stock Prices",
    width=600,
    height=150,
)